The invention relates to a crucible induction furnace provided with a preventive measure against low melting point metals.
There is a recent tendency for scraps or the like of galvanized steels and which are to be used in automobiles and washing machines to improve their corrosion resistance and which are to be melted in crucible induction furnaces.
FIG. 6 is a sectional view showing a main portion of a conventional example. When a steel containing zinc is melted at about 1500.degree. C. in a crucible induction furnace 1 that is made up of a crucible refractory 2 and an induction coil 3, the zinc 5 in the molten metal bath 4 is susceptible to permeation through the crucible refractory 2 due to the static pressure P.sub.S1 as shown in FIG. 6 , thus reaching the induction coil 3. As the reaching amount of zinc increases, the induction coil 3 may be burnt by the heated zinc 5 or, may, in the worst case, cause a hydrogen explosion due to its contact with water in a cooling coil.
To detect such undesirable conditions while charging a molten metal into the furnace, a molten metal leakage sensor is arranged on the inner surface of the induction coil 3. Such a sensor is disclosed in, e.g., Japanese Utility Model Unexamined Publications Nos. 101792/1988 and 182568/1987, and Japanese Utility Model Examined Publication No. 7278/1983.
The melting point of zinc is 420.degree. C. and its evaporating temperature is 920.degree. C., while the melting temperature of cast iron is about 1500.degree. C. Thus, in the crucible refractory 2 having a porosity of about 20%, it is likely that zinc in a gaseous state initially and in a liquid state as it permeates through the crucible refractory will eventually reach the outer side of the crucible refractory. Despite the fact that the crucible refractory 2 maintains its integrity without molten steel flashing, the permeation of the zinc therethrough causes the molten metal leakage sensor to operate erroneously or it burns and impairs the insulation of the induction coil 3 so as to reduce the refractory life.
In view of the above circumstances, a technique to check the permeation of low melting point metals is proposed in U.S. Pat. No. 4,989,218, in which gas passages consist of pipes providing holes and grooves inside the crucible refractory, and furthermore, consists of an additional lining of porous gas passages on the furnace wall, though not shown in the Figures.